Potassium channel is a protein which is distributed in the surface membrane of cells and selectively allows potassium ions to pass through it, and it is considered that it takes an important role in controlling membrane potential of cells. Particularly, in nerve and muscle cells, it contributes to the neurotransmission of central and peripheral nerves, pace making of the heart, contraction of muscles and the like by controlling frequency, persistency and the like of action potential. In addition, it has been shown that it is also concerned in the secretion of hormones, adjustment of cell volume, proliferation of cells and the like.
As the classification based on the opening and closing mechanism of the channel, a voltage-dependent potassium channel, an inwardly rectifying potassium channel, a calcium-dependent potassium channel, a receptor coupling type potassium channel and the like have so far been identified. In addition, an ATP-dependent potassium channel, a pH-dependent potassium channel and the like have also been reported. Among them, the voltage-dependent potassium channel has a characteristic in that it opens when the membrane potential is depolarized. In general, potassium ions are present in a non-equilibrium state of about 5 mM outside the cell and about 150 mM inside the cell. Thus, when the voltage-dependent potassium channel is opened by depolarization, potassium ions flow out from intracellular part to extracellular part and, as a result, induce restoration of the membrane potential (re-polarization). Accordingly, the opening of voltage-dependent potassium channel induces reduction of excitability of nerve and muscle cells and the like. Also, it causes changes in cellular functions in non-excitatory cells too, such as increase in the driving force for Ca.sup.2+ and subsequent increase in the flow of the same ion into the intracellular part. A compound capable of modifying opening of the voltage-dependent channel has a possibility of controlling various functions of cells, including excitability of nerve and muscle cells.
Genes of some types of the voltage-dependent potassium channel have been isolated from the brain and heart, and primary structure of the protein has been revealed. Based on the primary structure, it has been suggested that the voltage-dependent potassium channel has six transmembrane domains (S1 to S6) and one ion permeation region (H5). Also, it is assumed that the fourth transmembrane domain S4 contains basic amino acids having positive charge at intervals of 3 to 4 bases and functions as a voltage sensor.
These channels are roughly divided into Shaker type and eag type, based on the similarity of amino acid sequences. The Shaker type is a family having markedly high diversity and can be further divided into four groups of Kv1, Kv2, Kv3 and Kv4. On the other hand, the eag type is constituted by eag, eag-related gene and elk, and it related genes include hyperpolarization activation type potassium channels corresponding to KAT gene cluster and a cation channel which is activated by a cyclic nucleotide.
Regarding the importance of voltage-dependent potassium current in the brain, several findings have been obtained using these cloned voltage-dependent potassium channels. For example, relationship of Kv1.1 with memory and pain has been suggested by antisense-aided in vivo experiments (Meiri, N. et al. (1997) Proc. Natl. Acad. Sci. USA, 94, 4430-4434; Galeotti, N. et al. (1997) J. Pharmaco. Exp. Ther., 281, 941-949). Regarding Kv3.1, its participation in the excitability of GABA activating interneuron in cerebral cortex has been shown (Massengill, J. et al. (1997) J. Neurosci., 3136-3147). On the other hand, some experiments carried out using tetraethylammonium and 4-aminopyridine as non-selective inhibitors of the voltage-dependent potassium channel have also been reported. It has been shown that tetraethylammonium suppresses voltage-dependent potassium current in cerebral cortex nerve cells and also inhibits apoptosis of the same nerve cells (Yu, S. P. et al. (1997), Science, 278, 114-117). Also, it is known that intraventricular administration of 4-aminopyridine causes epileptic attack. These results suggest a possibility that an agent capable of controlling the activity of voltage-dependent potassium channel in the brain will become a therapeutic agent for central nervous system disorders such as dementia due to disturbance of memory and so on, nerve cell death accompanied by cerebral ischemia, epilepsy and the like.
On the other hand, most of the voltage-dependent potassium channels so far cloned are distributed in a large number of tissues among organs in the whole body. Thus, even when an agent which acts selectively on a specified voltage-dependent potassium channel is found, there is a possibility that the agent acts on many tissues and thereby induces originally unexpected agent effects. In order to find an agent having less side effects by targeting a potassium channel, it is necessary to clone a potassium channel in which its expressing tissue is restricted.